The single point mooring system for tankers, in which the tanker is moored to a single point while remaining free to rotate about the mooring point to align with the environmental forces and in which a cargo transfer means is integrated with the mooring system to permit continuous cargo transfer while the tanker rotates, has evolved during the past 15 years and is now the preferred mooring for very large tankers and deep water production fields. The primary requirements for such single point moorings are that they be a safe mooring and cargo transfer system at which mishaps resulting in damage and pollution are unapt to occur, and that they also be an efficient and economical system for which the costs of installation and operation are not excessive.
Most single point moorings now in use are of the catenary anchor leg mooring design comprised of a floating mooring buoy anchored by a number of catenary anchor chains connected to the periphery of the buoy which extend radially outward and downward to anchor points some distance from the buoy, as typically disclosed in U.S. Pat. No. 3,074,032. In deep water installations large and very long anchor chains are required and the cost of these chains is substantial, sometimes making such a mooring system excessively expensive. These systems further include floating cargo hose which connects to a cargo swivel mounted on the deck of the floating mooring buoy. The floating cargo hose experiences excessive wear at the point of connection between the hose and the buoy, and is exposed to damage should the tanker move forward and strike the buoy.
Some recent single point mooring installations which comprise more advanced single anchor leg mooring design are disclosed in U.S. Pat. No. 3,606,397 and 3,641,602, in which the mooring buoy is anchored by a single anchor leg and the hose connects to a submerged cargo swivel concentric with the anchor leg and mounted either on a mooring base or on a shaft pivoted on the mooring base. In this design the problem of excessive hose wear at the buoy connection and the danger of cargo system damage resulting from a tanker impacting the buoy are eliminated.
Further, in deep water installations the single anchor leg mooring has been proven to be less expensive than the catenary anchor leg mooring because the several very long anchor chains are replaced by a short anchor chain and a mooring base. The mooring base of the single anchor leg mooring may comprise a large hollow structure which is lowered to the ocean floor, filled with sand or other material to increase its mass to resist uplift, and pinned to the floor by piles or other means to resist sliding. In very deep water, installation of the base may prove difficult, and it may be advantageous to employ relatively short catenary chains to anchor the shaft through the center of the submerged cargo swivel. This is the basis of the present invention, which retains the advantages of the single anchor leg mooring.
In addition to the aforementioned prior art, reference also is made to U.S. Pat. Nos. 3,386,407 and 3,515,182. U.S. Pat. No. 3,386,407 discloses a mooring system for ships in which three or more catenary anchor legs extend directly from anchors on the sea floor to the side of the ship and are coupled at a point near the ocean floor by a ring which is slid down the anchor legs. An advantage of the present invention over the prior art is that it provides a cargo transfer system integral with the mooring system, thus allowing cargo transfer to continue while the tanker rotates around the mooring. U.S. Pat. No. 3,515,182 describes a mooring and loading system in which the mooring swivel surrounds the cargo conduit and in which the mooring lines extend directly to this mooring swivel to the tanker without the benefit of a mooring buoy. An advantage of the present invention over the prior art is that the mooring buoy supports the anchor hub above the sea bottom which provides for a more favorable mooring system elasticity, thus reducing the mooring loads. Furthermore, the present invention provides a relatively small compact mooring swivel which can better be designed to resist the mooring loads and to seal out water.